全新java初学者实践教程10(java SE5.0版)
11'Tt! jdk5的集合类
'f!Jh<i ;bbEd' ,1kV9_x !pXz-hxKT 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
(\_d'Js(; 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
a+Nd%hoe [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
A` 8If ]+S QS^4 [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
)FCqYCfk n(MEG'9} [font="Times](1) Set
sJvn#cS 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
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k ([font="Times]2)[font="Times]List
&ICO{#v5 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
lDXH<W? %;gWl1&5 G
0 yt%qHE [font="Times] ([font="Times]3)[font="Times]Map
q5Mif\ [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
1jb@nxRjO f#+ h_1# w[_Uv4M [font="Times] ([font="Times]4)[font="Times]Queue
_69\#YvCG [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
ivk|-C'\ [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
^u74WN q fe#k F9 vUA,` 实践:[font="Times] Set举例
}2{#=Elh [font="Times] import java.util.*;
Ks-><-2+N [font="Times]public class SetExample {
19DW~kvYk [font="Times] public static void main(String[] args) {
.j.=|5nVo4 [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
c eX*|B@= [font="Times] set.add("one");
BcWReyO<M [font="Times] set.add("second");
>oNs_{ [font="Times] set.add("3rd");
];YOP%2 [font="Times] set.add(new Integer(4));
03y<'n [font="Times] set.add(new Float( 5.0F ));
.?TVBbc%5 [font="Times] set.add("second");
\k8_ZJw [font="Times] set.add(new Integer(4));
5{[0Clb) [font="Times] System.out.println(set);
dWSH\wm+ [font="Times] }}
.BvV[`P 8a{g EZT, [font="Times]List举例:
6P8X)3CE<T [font="Times] import java.util.*;
o\#e7 Hqbh [font="Times]public class ListExample {
y.2 SHn0 [font="Times] public static void main(String[] args) {
N3)EG6vE* [font="Times] List list = new ArrayList();
.nJGxz+X" [font="Times] list.add("one");
<Th.}= [font="Times] list.add("second");
Z$@Nzza- [font="Times] list.add("3rd");
U# gmk0>t{ [font="Times] list.add(new Integer(4));
vbU{Et\^ [font="Times] list.add(new Float( 5.0F ));
!k^\`jMzw [font="Times] list.add("second");
+{Ttv7l_2 [font="Times] list.add(new Integer(4));
Qp}<8/BM\ [font="Times] System.out.println(list);
K9iR>put [font="Times] }}
G.8ZISN/ Vow+,,oh c2QC`h(Wb [font="Times]Map举例
C;|Ru* [font="Times]import java.util.Map;
2Qy&V/E ? [font="Times]import java.util.HashMap;
~}fpe>M: [font="Times]import java.util.Iterator;
|{(ynZ]R [font="Times]import java.io.FileReader;
z\, w$Ef+ [font="Times]
(J;<&v}Gad [font="Times]public class MapExample {
:1Ay_b_J [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
4T"P#)z [font="Times] Map word_count_map = new HashMap();
*(J<~:V? [font="Times] FileReader reader = new FileReader(args[0]);
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[font="Times] Iterator words = new WordStreamIterator(reader);
.W\Fa2}%av [font="Times]
Om*Dy} [font="Times] while ( words.hasNext() ) {
?p]w_l [font="Times] String word = (String) words.next();
(Y86q\DQ?| [font="Times] String word_lowercase = word.toLowerCase();
AiuF3`Xa [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
3-0Y<++W3> [font="Times]if ( frequency == null ) {
vnE,}(M [font="Times] frequency = new Integer(1);
3mWN?fC [font="Times] } else {
*hba>LZ [font="Times] int value = frequency.intValue();
sE% n=Ww [font="Times] frequency = new Integer(value + 1);}
_kfApO)O [font="Times] word_count_map.put(word_lowercase, frequency);
'" 6VfF)* [font="Times] }
MWB?V?qPSC [font="Times] System.out.println(word_count_map);
q-H]Hxv [font="Times] }}
gyIPG2d b.F2m(e2 RAvV[QkT [font="Times]Queue举例:
f-PDgs [font="Times]import java.io.IOException;
pLRHwL. [font="Times]import java.io.PrintStream;
TA*49Qp [font="Times]import java.util.LinkedList;
'sC{d&c [font="Times]import java.util.Queue;
LYT0 XB)A [font="Times]
'yl`0,3wV [font="Times]public class QueueTester {
-H{{ [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
$%/Zm*H [font="Times] public QueueTester() {q = new LinkedList();}
1mf_1spB [font="Times]public void testFIFO(PrintStream out) throws IOException {
(5SI!1N [font="Times] q.add("First");
)<+Z,6 [font="Times] q.add("Second");
) E5ax~ [font="Times] q.add("Third");
Xa36O5$4]9 [font="Times] Object o;
j&F&wRD%r [font="Times] while ((o = q.poll()) != null) {
9,4a?.*4~ [font="Times] out.println(o);}}
|m ~| [font="Times] public static void main(String[] args) {
0@2%pIq\ [font="Times] QueueTester tester = new QueueTester();
+ IMP< [font="Times] try { tester.testFIFO(System.out);
,ua]h8 [font="Times] } catch (IOException e) {
LGK}oL' [font="Times] e.printStackTrace(); } }}
xZ .:H&0G zk?lNs sD
M!Uv2n 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
`*!.B 总结:
nRvV+F0# 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
Upf1*$p 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。